The present invention relates to the formation of soft, flexible score lines on sheets of thermoplastic material, and in particular to a method and apparatus for simultaneously scoring and cutting blanks of plastic material for the production of boxes.
In providing plastic box blanks, a sheet of rigid or semi-rigid plastic material is cut to appropriate size and score lines are impressed therein so that the blank may be subsequently bent along these score lines into the form of the completed box. In forming the score lines on the blank, it is desirable that the score be a "soft" one, so that the plastic blank bends easily at each score line and when bent does not tend to return to its flat condition. It is also essential that the score lines be precisely and accurately positioned, and that their depths remain constant through lengthy manufacturing runs. The indentation of the score line must be of sufficient depth to enable the sheet material to be easily bent without cracking or crazing adjacent the score line, but must not be so deep that the material will split upon bending.
In the field of box making, methods have hitherto been devised for forming score lines in plastic box blanks, particularly blanks of transparent sheet material such as polyvinyl chloride. Initially, this was accomplished by the use of heated scoring blades applied to the surface of the plastic sheet. While these hot scoring blades produced score lines, they melted the plastic material and it was difficult, if not impossible to control their temperature, so that the depth of the score lines could not be maintained constant, the plastic tended to craze and crack adjacent the score, and the finished blank, when bent, would tend to return to its flat condition.
In U.S. Pat. No. 3,907,193, score lines are produced in plastic box blanks by applying heated scoring blades to the sheet material which rests upon a platen. Both the scoring blades and platen are electrically heated, with the result that, when the material is compressed between the platen and the scoring blades, stress relief indentations, constituting the score, are formed on the opposite faces of the sheet material. As with all externally-heated scoring blades, as described above, precise consistent depth of the score cannot be obtained. The score depths will also vary according to the type of material used and the thickness of the material.
In U.S. Pat. No. 4,064,206, a soft score is produced by pressing onto the plastic sheet material an edge forming tool which must be maintained at a temperature above the softening temperature of the plastic and below the melting temperature thereof. A high frequency electric field is applied through the plastic material causing the latter to soften by a dielectric heating action and to be deformed by pressure of the edge forming tool. This process produces commercially-acceptable score lines in most applications, but since it again depends upon the application of external heat, it is subject to the inherent deficiencies of such heat application. The apparatus employed is rather complex and expensive, requiring cooling ducts and a liquid coolant to control the applied temperature. In addition, because of the nature of the high frequency electric apparatus, it is possible to apply score lines on only one face of the material, and in some instances it is impossible to cut the blank simultaneously with the application of the score lines since the high frequency electric field tends to heat the cutting knives causing unwanted softening of the plastic in the vicinity of the cut. The process is also inapplicable to certain plastic materials having thicknesses below ten mils without producing separation at the score line, and also to certain co-extruded, laminated plastics composed of materials having different softening temperatures.
It is an object of the present invention to provide a process and apparatus for producing a soft score in plastic sheet materials without requiring sophisticated heating or cooling apparatus or the use of a high frequency electric field or ultrasonics. The score is provided by action of friction caused by pressing the material against a scoring die and vibrating either the material or the scoring die under pressure to produce frictional heat sufficient to form the score line.
Another object of the invention is the provision of a method and apparatus of the character described in which precise control of the depth of the score can be readily accomplished by simple regulation of the frequency of vibration, regulation of the pressure applied, and/or regulation of the size of the vibratory stroke.
Another object of the invention is the provision of a method and apparatus of the character described in which cutting and/or embossing of the sheet material blank can be effectively performed simultaneously with the formation of the score lines, and in which score lines can be applied simultaneously to both opposite faces of the plastic sheet material.
Still another object of the invention is the provision of a method and apparatus of the character described in which a satisfactory soft score can be produced in practically all thermoplastic materials regardless of their physical properties, and is equally effective in scoring materials of thicknesses below ten mils.
A further object of the invention is to provide a method and apparatus of the character described which is capable of incorporating standard type scoring tools in conjunction with vibratory apparatus which can be readily obtained at a relatively low cost compared to existing scoring apparatus depending upon heat or application of high frequency electric fields.